This invention relates to an orange to yellow emitting phosphor. More particularly, it relates to an orange to yellow emitting phosphor useful for a cathode-ray tube used in office automation equipments, and also to a cathode-ray tube manufactured by employing the phosphor.
As computers and the like have widely been used, various investigations have been made about the display for the cathode-ray tubes employed therefor. As for the displayed color hue, an orange to yellow display is desired for the purpose of preventing fatigue of eyes. Moreover, the fatigue of eyes is known to be caused by flicker phenomenon, whose generation depends on the fluorescence persistence properties and on the electrical excitation modes, even if the color hue may be satisfactory.
The flicker phenomenon will tend to be generated as the number of frames per second (frame frequency) is lower, and therefore displays may be sometimes practically useless in Europe where the frame frequency is lower (50 Hz) than those in Japan and the United States (60 Hz).
One means for resolving the above problem is to elongate the afterglow time of a phosphor. The other means is to make the frame frequency higher. It is disadvantageous to high-frequency display to design an electrical circuit to make the frame frequency higher. It is desirable to use long-persistence (or afterglow) phosphor.
As a phosphor material having the desired emitting characteristics as described above, there is a cadmium containing phosphor as disclosed in British Pat. No. 495,706. This may be represented by the chemical composition of Cd.sub.5 Cl(PO.sub.4).sub.3 :Mn. The time in which the fluorescent light intensity reaches 1/10 of the initial intensity after cessation of electron excitation (10% afterglow time) is as long as 25 to 30 milliseconds. In this case, even when the frame frequency may be as low as 50 Hz, it is possible to obtain a cathode-ray tube of an orange-emitting display without sensible flicker phenomenon.
In the above case where the frame frequency is about 50 Hz, the flicker phenomenon may sufficiently be alleviated if the 10% afterglow time of a phosphor is in the range from 10 to 100 milliseconds, particularly from 15 to 30 milliseconds. For this reason, the above-mentioned cadmium containing phosphor is practically useful.
However, cadmium contained in the above-mentioned phosphor is harmful to human bodies, involving so vital a defect that its use may sometimes be prohibited. Thus, the above-mentioned phosphor has a substantial problem.
In place of the aforesaid phosphor, as a phosphor emitting orange to yellow fluorescence with high color purity, it is possible to obtain a mixed phosphor wherein two or more kinds of emitting materials; for instance, europium-activated indium borate (red color) and terbium-activated indium borate (green color) disclosed in U.S. Pat. No. 3,394,084 may be candidates for these materials. This mixed phosphor has a specific feature in that its emission color can be freely controlled by choice of the materials to be mixed on the one hand, but on the other hand it also involves the drawback that a constant uniform emission color can be obtained only with great difficulties. That is to say, while it is possible to control the emission colors by varying the mixing ratio of two kinds of emitting materials with different emission colors, it is difficult to obtain a luminescent screen which is uniform in distribution of emission colors over the whole screen of the cathode-ray tube. For example, as shown by a in FIG. 1, different colored emission may occur partially in an edge band on the screen 1 of the cathode-ray tube. This may be prevented considerably, if its manufacturing process is controlled severely, but no complete prevention can be expected.
On the other hand, even if the mixing ratio of red (R) and green (G) of the phosphor particles may be fixed constant as shown in FIG. 2, the emission color obtained is not always constant when the component phosphor lots are changed, because size and fluorescent efficiency of both phosphor particles usually vary from lot to lot.
Further, it is also known to be difficult to obtain a high resolving power. Accordingly, it is impossible to attain the desired object of this invention by a mixed phosphor.
As a substitute for such a mixed phosphor, it has now been thought of to use a Eu and Tb co-activation type phosphor. In this type of phosphor, activators added to respective phosphor particles was expected to maintain sufficiently the emission characteristics of the activators to give an emission of high efficiency. Moreover, it was also expected that the emission color could freely be controlled by the concentrations of the respective activators. However, these expectations failed to come true, because the emission characteristic of the phosphor co-activated with Eu and Tb at concentrations preferable for single activations proved to be greatly different from that expected. That is, the emission color became reddish orange. Moreover, the emission efficiency was found to be lowered to a great extent as compared with the singly activated phosphors.
The present inventors began to carry out further extensive studies about utilization of the advantage of the above-mentioned co-activation and consequently found that the object of this invention could be accomplished only within a limited range of materials and activator concentration.
An object of this invention is to provide an orange-yellow emitting co-activated phosphor which can give a desired emission color between orange and yellow and which does not include a harmful element in its chemical composition, and which can also provide a luminescent screen having excellent color purity, sufficiently long afterglow and high resolution.
This invention will be described in detail below with reference to the accompanying drawings.